@tbk: Nyx (prime) (chaos gives you a lot of breathing room), Brakk (corrosive build for heavy gunners), Dual Ichors (primarily for coptering). Didn't really put any special effort into anything but the Brakk build.
I forget exactly how much forma I've sunk into what (3 for Brakk, I think?)
I brought a Grinlok (rank 4) for leveling (bad idea, barely got any affinity) and a Helios for scanning (because I'm lazy).
I'm thinking I have a lot of room to optimize my build to go further. Probably a bow or status weapon to faster take down gunners at higher levels, which probably means I'm better off bringing something with more range than Brakk, though I am very familiar with that weapon. And, of course a proper sentinel.
Tempted to bring my 6x Forma Penta. But I do have a tendency to suicide with it, so... probably best to stay away from the explosive weapons. (edit: just realized that there's probably a lot of synergy between nyx/absorb and penta, hmm...)
Stopping at any point past ~15 minutes is suicide, I noticed, as I had a couple of random deaths in areas I thought were safe (hiding in nooks to use a health restore occasionally, when I'd gotten a bit bold)